Group delay adjuster circuits play a key role in a number of applications which require compensation for group delay, such as feedforward power amplifiers, and the like. In an exemplary feed forward amplifier, a group delay adjusting circuit might be used in loop compensation circuitry such as a phase and gain adjuster. Alternatively, the group delay adjusting circuit may be implemented as a separate functional block prior to the phase and gain adjusting circuit.
In an electrical network transmission without distortion is typically achieved when an amplitude and a group delay response of a network are as close to constant as possible. If an electrical network has a non-constant group delay, group delay compensation in the form of an electric compensation, or group delay adjusting, circuit may be added so that the overall electrical network response is more nearly constant.
As will be appreciated by those skilled in the art the frequency response of a network may be represented as a function of a frequency T, by F(jT)=A(jT)+B(jT) or equivalently as F(jT)=*F(jT)*ejN(T). The magnitude of the amplitude response of the network is defined as *F(jT)*=[A2(jT)+B2(jT)]1/2. The phase angle of the network is defined as N(T)=tan−1[A(jT)/B(jT)]. The group delay of the network is defined as θ(T)=dN(T)/dT. Two signals having an equal propagation delay have equal phase verses frequency slopes. Two signals have constant delay when the phase verses frequency slope, or group delay is constant. The group delay response of a the network is the response that is sought to be compensated for with a group delay adjusting circuit so that an overall group delay response for the composite response of the two networks tends to be flattened, and thus compensated for. Feed forward power amplifiers amplify multiple carriers, or groups of frequencies.
In a patent issued to Kondo et al. (U.S. Pat. No. 5,146,192) a delay circuit comprising a variable capacitance diode and an inductor is disclosed. In this circuit, an input signal is applied to a series inductance that is made up of discrete series inductors. The inductors of this circuit are not variable, but fixed in value. A series of shunt capacitance comprising diodes are coupled at the node where each of the series inductors are joined. The parallel capacitance is formed from two diodes that are connected cathode to anode at the junction of each pair of inductors to cause each diode to form an equal capacitance, equal and opposite signal voltage are applied to each diode. The capacitance of each variable capacitance diode (varactor) is adjusted by applying a control voltage of the same magnitude to each of the diodes. The control voltage applied to each varactor diode is equal in amplitude and opposite in polarity. Thus, a single control voltage is applied to the circuit, but the circuit must provide a way of reversing the polarity. Those skilled in the art will realize that, this may be accomplished by providing a circuit such as an inverting amplifier to respective varactor diodes.
A patent issued to Bock et al. (U.S. Pat. No. 4,189,690) discloses a resonant linear frequency modulator. As shown in FIG. 1 of this patent, an oscillator is coupled to a modulator circuit through a matching circuit. Where the matching circuit provides the required mismatch between the modulator and oscillator needed to maintain oscillator modulation. The modulator is biased by two DC signals VB and −VB that do not vary or change. These DC signals are applied through radio frequency (RF) chokes implemented in quarter wavelength microstrip line to prevent RF signals from leaking into the power supplier coupled to the bias port. The bias ports also include shunt bypass capacitors selected such that any energy from radio frequency signals passing through the RF choke is shunted to the ground by the low impedance of the capacitor at RF frequencies. This prevents contamination of the power supply or bias supplies.
The modulator includes three reactants elements, a varactor diode (34 of FIG. 1), a varactor (36 of FIG. 1) and an inductor (42 of FIG. 1). Varactor 36 and inductor 42 are made to appear as an inductance to the oscillator circuit through a quarter wavelength transmission line section 38 disposed between varactor 34 and varactor 36. The impedance seen by the oscillator is zero with no modulating signal applied at the modulating signal input port Vm. Both the inductive and capacitive reactances are changed in an equal amount by the single modulating signal that is applied to the modulator.
The modulating signal is used to change the frequency produced by the oscillator in proportion to the modulating signal, such as typically occur in FM modulation. The circuit is not used to adjust group, or envelope delay of a band signals as would be done with a group delay adjusting circuit. As shown, the output 13 is an FM signal.
The bias voltages are applied or substantially identical and opposite in polarity, and serve to back (reverse) bias the varactors. When the single modulating voltage input is applied to this circuit, a reactive imbalance is formed at opposite ends of the quarter wavelength impedance transforming line (38 of FIG. 1). The modulating voltage operates in a push/pull manner to apply a capacitive or inductive reactance to the oscillator. The oscillator and matching circuits have been designed for low Q (quality factor) and are thus sensitive to changes in reactance applied by the modulating circuit. The oscillator frequency is caused to change due to the change in reactance of the modulating circuit. Thus, the single modulating voltage causes the frequency modulation of the oscillator to produce a frequency modulated signal.
A patent issued to Seino (U.S. Pat. No. 6,400,237) discloses a phase compensation circuit. In order to provide phase compensation, the circuit disclosed in this patent utilizes a parallel combination of an inductance and a capacitance, where the capacitance is provided by a varactor diode. In an embodiment having a variable inductor, the inductor is varied by changing the length of its circuit path, which is a time-consuming operation that must be performed by hand (see FIGS. 5A and 5B of the Seino patent). This type of inductance is not electronically tunable and inherently not suitable for use in a dynamically adjustable group delay adjusting circuit.
Another embodiment shown in Seino utilizes a quarter wavelength transmission line coupled to a main transmission line having a variable capacitance disposed on the opposite end of it. (See FIG. 10 of Seino) However, this arrangement utilizes only a single control voltage to adjust the impedance. The impedance adjusted typically varies between the capacitive and an inductive value depending upon the tuning voltage. Missing from this arrangement is a parallel capacitance that may be independently tuned.
Often it is desirable to match the characteristics of signal paths to optimize electrical performance. For a feedforward amplifier to be effective over a wide bandwidth in canceling distortion it is desirable to have cancellation loops with the greatest cancellation possible over the greatest bandwidth possible. For example, in a linear feed forward power amplifier (FFPA), one or more error correcting, or error cancellation, loops are present. In each loop a signal will typically travel through an active signal path present in active circuits and a passive signal path through the passive circuits. Loop cancellation tends to be optimal when signals traveling over the active and passive signal paths tend to have equal amplitude responses, opposite phase responses, and equal group delay.
Compensation is typically provided by a network having inductors and capacitors disposed in it to achieve a compensating response. Inductors are usually difficult to build, and often require trimming and/or adjustment. In particular mechanical variable inductors are difficult to build, and particularly unsuited to high frequency applications. For example see U.S. Pat. No. 6,400,273 describing a mechanically adjustable variable inductor, where the inductance is varied by varying the position and length of a thin metal line. Other types of inductors are typically implemented for example, by a coil of wire wound on a form, or a spiral of foil disposed upon a substrate or printed circuit board, and typically provided with some sort of mechanical adjustment to vary the inductance.
For example a variable inductor is described in U.S. Pat. No. 5,999,077 issued to Hammond, et al. that describes a voltage controlled variable inductors using a variable air gap to control inductance. Those skilled in the art will appreciate that air/transformer gap variable inductors are extremely bulky, expensive and limited in frequency response due to the physical construction and material used. It is desirable to imitate such an electrically variable inductor or the like by providing a component that produces a negative reactance, without utilizing any of the conventional inductor components. Such a novel component could be said to produce a negative reactance, or inductance virtually. That is a “virtual inductor” that is able to produce a variable inductance (or negative reactance), without the use of a conventional variable inductor component is desirable.